Progress and prospects of variable load fracturing characteristics

Hydraulic fracturing plays a crucial role in the development of unconventional energy resources. However, traditional hydraulic fracturing faces many challenges, such as excessive breakdown pressure, potential geological risk of fault slip, and pollution of the environment by acid fracturing fluids, etc. How to achieve efficient reservoir reformation on the premise of reducing the risk of inducing natural disasters is a key scientific topic, and the new fracturing methods were needed urgently to deal with these challenges. Recently, variable-load fracturing, as a form of fracturing operation that applies periodic or random variable loads (such as pressure, fluid, temperature, and high-energy gas, etc.) acting on formation rocks through physical or chemical means, has gradually attracted widespread attention. Variable-load fracturing emphasizes the multi-field coupling effect between factors such as fluid, temperature, high-energy gas, etc. and the porous medium of rocks during the fracturing process, which is fundamentally different from the mechanical fatigue failure of metals and has a broader conceptual scope. This paper systematically reviews the latest advancements in variable-load fracturing experiments and comprehensively summarizes the findings from four key perspectives: research subjects, influencing factors, research contents, and analytical methods. The variable-load actions are categorized into six primary types: cyclic load, increasing load, stepped load, pulse load, impact load, and combined load. Particular attention is given to the specific effects of variable-load on rocks both before and after fracture, summarizing four characteristics of variable-load fracturing: reduction in rock fracture pressure, decrease in rock fracture energy, alteration in rock permeability, and generation of complex fracture patterns. Furthermore, this paper delves into the underlying mechanisms of variable-load fracturing, examining aspects such as rock fatigue fracture, stress corrosion, and fracture propagation in deflagration fracturing. Finally, the future research directions on variable-load fracturing characteristics were emphasized. It is believed that the influence of rock structure characteristics on the permeability enhancement effect and complex fracture morphology should be clarified. Keeping up with the research trends in deep/ultra-deep reservoir development, the applicability and potential changes of variable-load fracturing under complex and extreme conditions should be considered thoroughly to broaden the research and development space for variable-load fracturing. The action characteristics and internal mechanisms of variable-load fracturing should be comprehensively analyzed from the perspectives of multiple fields, multiple scales, and multiple time periods. This paper aims to provide a comprehensive basic understanding of variable-load fracturing and offers ideas and directions for future research and development.